Throw when reflecting on elements not covered by a `MirrorsUsed` annotation.

tests/compiler/dart2js_extra/mirror_invalid_field_access4_test.dart

+// Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// Test that we cannot reflect on elements not covered by the `MirrorsUsed`
+// annotation.
+
+library test;
+
+@MirrorsUsed(targets: 'C.foo')
+import 'dart:mirrors';
+
+import 'package:expect/expect.dart';
+
+class C {
+  var foo;
+  var bar;
+}
+
+class D {
+  get bar {}
+  set bar(x) {}
+}
+
+int inscrutable(int x) => x == 0 ? 0 : x | inscrutable(x & (x - 1));
+
+main() {
+  var c = inscrutable(1) == 1 ? new C() : new D();
+
+  c.bar = 1;
+  var local = c.bar;
+
+  var mirror = reflect(c);
+  Expect.equals(1, mirror.setField(const Symbol('foo'), 1).reflectee);
+  Expect.equals(1, mirror.getField(const Symbol('foo')).reflectee);
+  Expect.throws(() => mirror.setField(const Symbol('bar'),  2),
+                (e) => e is UnsupportedError);
+  Expect.throws(() => mirror.getField(const Symbol('bar')),
+                (e) => e is UnsupportedError);
+}